Introduction to Marine Seismic Processing - ProMAX Andrew Goodliffe University of Alabama Socorro, NM, Wednesday May 28

ProMAX Exercise

•What is ProMAX?

•A software package for processing reflection seismic data •Commonly used in the energy industry •Not free!

•There are many other programs that will do the same sort of thing – they differ mainly in their user interface (or lack thereof) •Runs on many flavors of UNIX

Where did the data come from?

• A short seismic reflection survey in Papua New Guinea • September 1999 • R/V Maurice Ewing – 1.2 km streamer – 48 channels – 25 m group interval – 1395 inch

3

tuned six airgun array – 25 m shot interval – 24-fold CMPs 12.5 m apart • 1 Arc-second (~30 m) grids

Woodlark Basin

•A rift basin in Papua New Guinea •A classic place to study the orogenic rifting to seafloor spreading transition •The line that we will look at crosses a large rift basin close to the transition to seafloor spreading •The survey was carried out as part of an Ocean Drilling Program site survey

High Pressure Air Sources: The Air Gun

Ready

Lower chamber has a top diameter that's smaller the bottom diameter air pressure forces the piston down and sealing the upper, firing chamber. High pressure air is filling the firing chamber through the T shaped passage, and the firing, or actuating air passage is blocked (solid black) by a solenoid valve.

Fire!

Full pressure has built up in the upper chamber. The Solenoid has been triggered, releasing high pressure air into the active air passage, which is now yellow. The air fills the area directly below the piston, overcoming the sealing effect of the air in the lower, control chamber. The piston moves upwards, releasing the air in the upper chamber into the water.

Fired

A large bubble of compressed air is expanding into the surrounding water. The air in the lower control chamber has been compressed. The triggered air, released into the space below the piston, is fully expanded, and can now exhaust at a controlled rate through the vent ports. As this takes place, the piston rapidly but gently moves downward, re-sealing the chamber, and readying the sound source for refilling.

Air Guns

•Airguns suspended from

stowed booms Other source?

•Single Air gun – note air

ports

Tuning An Air Gun Array

•Summing the signal of multiple guns creates a more desirable

signal

•Note the relative scales of the left and right plots

Listening

• Hydrophone – Piezoelectric material – Pressure changes in the water generate small currents which

are amplified

• Geophone – Mechanical – Motion of coil relative to magnet generates a small current

which is then amplified

How is a Marine Seismic Reflection Survey Shot?

Definition of shot and common mid-point (CMP) gathers Shot gather: All the data recorded on all the channels by a single shot CMP gather: A collection of traces that have been recorded at the same location.

Shot and CMP gathers are simply different ways of sorting the data.

What is the natural CMP spacing relative to the group interval?

Starting ProMAX

• Type Promax on the command line • Select the survey • Select the line that includes your name

Anatomy of ProMAX

Click on the

EW9910

area, then

EW9910 Line ODP11

This area tells you what each of your mouse buttons will do – ProMAX uses three mouse buttons List of flows – operations that we will apply to the data Status of any jobs that are running

Anatomy of ProMAX

Click on 01 – Display Shots This flow reads in the seismic data and displays it List of individual processes – things we can do to the data. Flows are built up of a sequence of processes. Click on one and it will appear in the left-hand window. Delete and processes accidentally added by clicking on delete in the left-hand window

Anatomy of ProMAX

Click on Disk Data Input with the middle mouse button. This will parameterize the process. Here we see that we are reading in

11 Shots w/geom

(raw shot file with navigation added to the headers), sorting the data by

Source index number

(shot number), reading in every 10 th shot 300 to the end of the file shot from

Anatomy of ProMAX

Clicking on the data file

11 Shots w/geom

with the left-hand mouse button (LHMB) will take you to a list of data files. Click on

11 Shots w/geom

with the middle mouse. The details of the data file are now displayed We can now see how many traces there are in the file, the sample rate (in milliseconds), how many samples there are per-trace, the minimum and maximum CDP, and the minimum and maximum shot (

SIN

). Moving your cursor to the top of the screen will take you back to the flow

Anatomy of ProMAX

Clicking on the data file

11 Shots w/geom

with the left-hand mouse button (LHMB) will take you to a list of data files. Click on

11 Shots w/geom

with the middle mouse. The details of the data file are now displayed We can now see how many traces there are in the file, the sample rate (in milliseconds), how many samples there are per-trace, the minimum and maximum CDP, and the minimum and maximum shot (

SIN

). Moving your cursor to the top of the screen will take you back to the flow

Displaying a Shot

Clicking on Execute with the LHMB Direct ray path – sound travels directly from the airgun array to the hydrophones – forms a straight line Reflected ray path – sound bounces of the seafloor and underlying layers – forms a hyperbola Water column noise

Water Velocity

Clicking on the zoom icon. By holding down the LHMB and dragging a box, zoon into the area where we see the direct wave. The gradient of the direct wave gives us the water velocity. Click on the gradient icon. By holding down the LHMB, drag a line that follows the first arrival of the direct wave. The corresponding velocity will be displayed at the bottom of the screen Which channel is nearest to the ship?

Zoom icon Gradient icon

Near-Trace Plot

When we are collecting data we want to see it as quickly as possible – one way of doing this is by displaying a near-trace plot. This is simply a display of the channel nearest to the ship for each shot. This will give us the first glimpse of what we are looking at in terms of geology. Go back to the list of processes and click on 02 – Near Trace Plot. Execute the flow.

Seafloor Graben bounding faults Basement Multiple

Near-Trace Plot

Go back to the flow

02 – Near Trace Plot

and uncomment

Automatic Gain Control

by clicking on it with the right-hand mouse button (RHMB). This will add gain to the section, enhancing the deeper reflectors

Power Spectrum

Go back the list of flows. Click on the flow

03 – Power Spectrum

and execute it. This flow is setup to show the frequency content of every 10 th shot. We use a plot like this to determine characterize the range of frequencies in data, and possibly identify noise Click on the arrow to go to the next shot Shot gather Frequency content by channel Frequency range Phase

Filtering

We can use a bandpass filter to remove frequencies below and above a certain range. We are now going to test some filter parameters using the process

04 - Filter

The filter defined in

Parameter Test

will remove all frequencies below 6 Hz and above 80 Hz. All frequencies between 10 and 70 Hz will be kept. A ramp is applied to intermediate values The number the

99999

next to filter values indicates that the actual filter value comes from

Parameter Test

process Execute the flow

Filtering

For each shot a filtered and unfiltered ( • • Zoom in to look at the data in detail Try some different filters

Control copy

) version of the data is displayed. Advance to the next shot by clicking on the arrow.

Removing NMO

The reason for having so many (24 in this case) traces in a CMP is so that we can stack (sum) the traces for a given CMP.

• Noise cancels out • Real signal (geology) is amplified • Signal to noise ratio increase • First we must remove Normal Moveout (NMO) – the difference in travel time that is the result of varying ray path lengths 

T V

 

t x



t

0

x

2

t

0 

T



x

2 2

V

2

t

0

CMP

Removing NMO in Practice – Velocity Analysis

Semblance plot Nowadays velocity analysis is carried out using semblance plots – these show how well the data stacks (i.e. a reflector is coherent across a stack after NMO is applied) for a given two-way travel time and velocity NMO has been removed correctly and the reflector is now coherent Go back to the

flows

list in ProMAX and select

05 – Velocity Analysis

– click on

Execute

Velocity Analysis

Click on the zoom icon and zoom into this area Dynamic stack Semblance plot CMP

Click on the pick icon to pick velocity/ti me point on the semblance plot Click on

Gather

–

Apply NMO

to see NMO applied as you pick

Velocity Analysis

NMO Add velocity/time points to the semblance plot such that the NMO is removed for the major reflectors. • Zoom in and out as necessary • Do not pick the multiple • Save your picks

Stacking

Go back to the list of flows.

• Click on

06 – Stack

• This flow uses your velocity picks and other that were picked earlier to stack the data • The traces in each CMP are summed to form one trace Removes some residual noise and spikes Applies a bandpass filter to the data Traces in each CMP are stacked Execute the flow – this will take some time…..

Applies the NMO correction using the picked velocities Trace mutes to remove stretched traces and attenuate multiple

View Stack

Go back to the list of flows.

• Click on

07 – View Stack

• Execute the flow • You will see that the image is now much better than our original near trace plot • You can start to see stratigraphy • However, the are lots of diffractions and reflectors are not in their correct subsurface location – we need to migrate

subsurface geometry.

Migration

In an un-migrated time section reflectors do not represent the true • See examples below… (A) (C) Seafloor (B) Bow-tie effect Geological Cross section Time section Time section Dipping reflectors (A) a syncline on the seafloor is imaged as a “bow-time section (B) The addition of diffractions from the end of reflectors results in a very complex time section (B) A dipping reflector is shallower in a time section

Go back to the list of flows.

• Click on

08 – Migration

Migration

Using a velocity model that was made earlier we will migrate the data There are a number of ways of migrating the data – all are mathematically very complex….

Execute the flow. This will take some time When it has finished running, Click on

09 – View

Migration and execute the flow

View Migration

• Most diffractions have gone • Geology is now gar more evident • Remaining problems: smiles; frowns • Solution: improve velocity model; more advance processing • Why is this still not equivalent to a geological cross-section?